1. Field of the Invention
The invention relates generally to a fluidized bed heat exchanger system and more particularly to a fluidized bed heat exchanger utilizing specifically designed apparatus to stabilize particulate matter porosity and primary medium fluid flow within the system and improve inner tube surface cleaning and a method for operating same.
2. Description of the Prior Art
There are numerous ways to effect heat transfer between two mediums. Two of the more common means to accomplish such a heat transfer are a medium-medium contra flow heat exchange or a fluidized bed heat exchange. The present invention relates more generally to the fluidized bed heat exchange method than to the medium-medium contra flow method.
Conventional fluidized bed heat exchanger systems pass a primary medium, to be heated or cooled, through a plurality of tubes. These tubes are bathed in a secondary medium, selected to cool or heat the tubes. Fluidized particulate matter, such as glass beads or chopped wire, may be suspended in the flow of the primary fluid to enhance the heat exchange and scrub the inner surfaces of the tubes.
When fluidized particulate matter is utilized in a heat exchange system the particulate matter forms two beds or regions of concentrations within the heat exchanger. The first bed develops at the base of the tubes and surrounds the open base ends of the tubes. Due to the primary medium fluid flow through the first bed a second particulate matter bed develops within each tube. Optimum primary medium fluid flow and heat transfer require that these two particulate matter beds be stable. Maldistribution in the primary medium fluid results in reduced heat transfer between the primary and secondary mediums adversely affecting the system's efficiency. Under certain conditions particular tubes, due to poor primary medium fluid distribution, accumulate larger than desired amounts of particulate matter and develop into unintended "downcomers". These downcomer tubes allow primary medium and particulate material to back flow through the heat exchanger system. This back flow through the downcomers also adversely affects the heat transfer characteristics of the heat exchanger system.
Particulate matter bed stability is enhanced if the density of the particulate beds surrounding the base of the tubes is greater than the density of the particulate matter bed within the tubes and further if the density of the lower regions of the particulate tube beds is greater than the upper regions of the particulate tube beds. Particulate matter bed stability is further increased if the degree of density change between the beds and the regions within the tubes is of a gradual nature.
In some applications it is not necessary to maintain constant fluidization of the particulate matter within the heat exchanger and higher primary medium flow rates can be achieved if the fluidized particulate matter is only present within the system when tube descaling is necessary.
The use of a fluidized bed heat exchanger systems is known in the prior art. Such systems are described in U.S. Pat. Nos. 1,716,333; 3,886,997; 3,991,816; 4,119,139; 4,220,193; and 4,300,625 which disclose the use of fluidized particulate matter in heat exchanger systems.
The majority of the aforementioned patents disclose different designs and apparatus to minimize undesirable internal particulate disturbance in the aforementioned particulate matter beds. U.S. Pat. No. 4,119,139 specifies the use of throttling devices within the tubes of the heat exchanger to affect particulate stabilization. U.S. Pat. No. 3,886,997 discloses the use of fluidized particulate matter of specified size and design to control particulate distribution and reduce high pressure loss. U.S. Pat. No. 4,220,193 prescribes the use of a perforated sieve plate and inlet devices, with lower edges perpendicular to the central axis of the device, attached to the bottom of the tubes as a means to control particulate distribution in the fluidized bed heat exchanger system. U.S. Pat. No. 4,300,625 teaches the use of specially contoured housings in conjunction with prescribed amounts of fluidized particulate matter as a means to stablize particulate disturbances and increase inner tube cleaning.
U.S. Pat. No. 3,991,816 discloses a method for cleaning a fluidized bed heat exchanger. The disclosed method teaches the use of expensive auxiliary apparatus to implement a cleaning procedure and requires interruption of the heat exchanger's operation to utilize the descaling procedure.
One disadvantage of the conventional fluidized bed heat exchanger system relates to the generation of instability in the particulate matter beds resulting in non-uniform primary medium flow rates through the system, adversely affecting the heat exchanger ability to process large quantities of the primary medium over a wide range of primary medium flow rates.
Another disadvantage of the conventional fluidized bed heat exchanger concerns the development of unintended downcomers among the tubes resulting in less than optimal heat transfer between the primary and secondary medium.
Another disadvantage of the conventional fluidized bed heat exchanger system concerns the inability of prior designs to function without distributor or sieve plates. Such plates in certain applications are susceptible to clogging, thus, preventing primary medium flows through said plate and generating turbulence or unintended preferential primary medium flows within the particulate matter bed situated about the distributor or sieve plate. Such preferential flows and turbulence, as discussed, reduce the heat exchanger efficiency thereby decreasing the cost effectiveness of the system.
Another disadvantage of the conventional fluidized bed heat exchanger system concerns the inability of the present systems to use a method for regulating the amount of fluidized particulate matter utilized within the heat exchanger system without interrupting the operation of same.